Biology Reference
In-Depth Information
x
A. cardenasii
introgression line and a culti-
vated variety. Two breeding lines developed from
this material have been placed into advanced line
trials.
Mapping of RFLP markers on BC
3
F
1
lines
in greenhouse studies identified five markers for
leafspot resistance (Burow et al. 2008), including
three QTLs for incubation period and one each
for latency period, lesion number, and diame-
ter. Those QTLs for latency period and lesion
number were overlapping, suggesting linkage
between the two or a QTL with pleiotropic
effects. In addition, field evaluation of BC
3
F
2
lines identified 29 markers for the domestication-
related traits of main stem length, number of lat-
eral branches, and pod and seed size (Burow et al.
2011).
Leal-Bertioli et al. (2009) reported the map-
ping of 34 RGAs and 5 QTLs for late leaf
spot disease resistance on detached leaves of the
F
2
plants of the A-genome mapping population
derived from
A. duranensis
x
A. stenosperma
,
and suggested additive or partial dominance
gene action. One QTL explained almost half
of the phenotypic variance observed. Two QTLs
mapped near RGA markers. In a detailed QTL
study based on cultivated genotypes, Khedikar
et al. (2010) reported 11 QTLs for LLS; each
QTL explained 2-7% of phenotypic variation in
three environments, suggesting that the genes
controlling LLS resistance in this cross are rel-
atively minor. In maps from two populations,
again using GPBD-4 as one parent, using a
larger number (188 and 181) of markers and
six trials, a major QTL for LLS was reported,
which explained from 10% to 62% of phe-
notypic variance, depending on the environ-
ment; this appeared to give a bimodal (resis-
tant/susceptible) distribution (Sujay et al. 2011).
In all, 28 QTLs for LLS were identified.
These findings add to several others that leaf
spot resistance in peanut is under the control
of many genes and thus explains the difficulty
in breeding for resistance. However, identifica-
tion of a major QTL may allow for more rapid
progress in transferring a significant degree of
resistance from donor populations. Fonceka et al.
(2009) concluded that the BC
1
F
1
and BC
2
F
1
interspecific hybrids resulting from their work
should facilitate the development of advanced
backcross and chromosome segment substitu-
tion breeding populations for the improvement
of cultivated peanut, having used the putative
progenitors of cultivated peanut from both the
A and B genomes for the development of their
interspecific amphidiploid. Combination of
QTLs for agronomic and quality traits with those
for leafspot analysis is expected to significantly
accelerate breeding for resistance.
Rust Resistance: A Complex Trait that
Could Be Simpler to Breed than
Thought
Etiology
Rust (caused by
Puccinia arachidis
Speg.) is
another important fungal disease that occurs
widely in Africa and Asia and sporadically in
North America and South America. It appears as
a large number of small pustules on the underside
of leaves, and in severe cases can cause signif-
icant defoliation and loss of yield. Overall, rust
is generally less significant than leaf spots, even
though occasionally outbreaks are severe and can
cause severe losses. Rust frequently occurs in
combination with leaf spots. Yield loss due to
rust, in combination with early and late leaf spot
diseases, can be particularly severe; in India, it is
reported to be as high as 70% (Subrahmanyam
et al. 1980, 1985).
Breeding
Resistance to rust, as also is the case for resis-
tance to leaf spots, has been considered to be a
quantitative trait. Resistance is measured as sev-
eral components: leaf area damage percentage,
infection frequency, incubation period, lesion
diameter, and sporulation index. All measures
were found to be positively correlated with one
another, with the exception of incubation period,
which was negatively correlated with the other
measures (Mehan et al. 1994).
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